One process to produce methacrylic acid is the catalytic carbonylation of propylene to give isobutyric acid followed by partial oxidative dehydrogenation to yield methacrylic acid. The crude product from this process is a mixture containing water, isobutyric acid, methacrylic acid, and other components. The water is removed using either azeotropic distillation or solvent extraction followed by distillation to remove the extraction solvent and produce an anhydrous mixture. Methacrylic acid is typically separated from the anhydrous mixture using either distillation or crystallization techniques. In separations involving distillation the physical and chemical property similarities of methacrylic and isobutyric acids typically require a large number of theoretical plates in combination with high reflux ratios to achieve high product purity. Furthermore, use of distillation as a separation technique is complicated by the tendency of methacrylic acid to polymerize while being distilled.
When crystallization techniques are employed in the separation, the combination of low melting point for isobutyric acid and low eutectic temperature for isobutyric/methacrylic acid mixtures require that very low crystallization temperatures be used. Such temperatures made crystallization processes difficult to practice economically.
Japanese Kokai 62-145044A describes a distillation method to obtain purified methacrylic acid through the use of at least four separate distillation towers and an extraction column to remove and purify an extraction solvent and to remove both high and low boiling impurities. Japanese Kokai 52-007917A describes a procedure for separating methacrylic acid from the crude aqueous product of the gas phase dehydrogenation of isobutyric acid in which the crude product is first distilled to remove materials with boiling points less than water and then extracted with a hydrocarbon solvent to remove methacrylic acid and related materials. The solution of hydrocarbon solvent, methacrylic acid, and related materials is distilled stepwise, first to remove residual water and then to remove the hydrocarbon solvent. This process results in a methacrylic acid product composition which is about 97% by weight methacrylic acid.
U.S. Pat. No. 4,780,568 describes purification of an anhydrous mixture of methacrylic acid, isobutyric acid, and other impurities using a staged crystallization separation unit with 3-6 equivalent stages and a secondary recovery section with one or more stages to give high purity methacrylic acid. This process requires very low crystallization temperatures. Russian Patent no. 639,858 describes purification of acrylic and methacrylic acids using a countercurrent flow crystallization process involving low temperatures. N. Wynn in Chemical Engineering Progress, 88(3), 52-60 (1992) describes the use of distillation and crystallization together in order to improve product purity. Distillation is used to remove the bulk of the impurities followed by melt crystallization to obtain the final high purity product.
Because these known processes are often costly; difficult to control, and/or provide a less pure product than often desired, there continues to be a need for improved methods to efficiently and cost effectively separate methacrylic acid from mixtures containing isobutyric acid.